Yellow Fever Vaccine-Related Neurotropic Disease in Brazil Following Immunization with 17DD

The disease burden of yellow fever virus infection (YFV) is quite high in the tropics where vaccination coverage is low. To date, vaccination is the most effective control strategy to mitigate and eliminate the burden of YF disease. The licensed YF vaccines are safe and effective and serious adverse events are rare. Herein, we report three cases of neurological syndrome, compatible with meningoencephalitis following 17DD vaccination. In all cases, YFV-specific IgM antibodies were detected in the cerebrospinal fluid. Our observations confirm the development of YF vaccine-associated neurotropic disease, a rare serious adverse event, from which all three patients have fully recovered without any long-term sequelae. This report reinforces the need for awareness among health professionals to recognize and effectively manage such events in a timely manner.


Introduction
During the last decade, several yellow fever virus (YFV) outbreaks were reported in Western, Central, and Eastern Africa, and South America [1]. Clinical manifestations of infection present with a wide clinical spectrum, ranging from a mild acute febrile flulike disease to a severe and fulminant hepatorenal syndrome with high morbidity and mortality [2,3].
Currently, there are two live attenuated yellow fever vaccines in use worldwide (17D-204 and 17DD), which both originated from the 17D vaccine developed in 1937 [3]. In 2019 and 2020, the YF vaccines supply available worldwide was 135 and 150 million doses, respectively, and will likely increase due to the Eliminate Yellow fever Epidemics ("EYE") global initiative for eliminating the disease by 2026 [4]. Although both vaccines are safe and effective, rare serious adverse events (SAEs) have been reported over the years [5]. They are classified as (i) hypersensitive reactions, (ii) YF vaccine-associated neurological disease (YEL-AND), and (iii) YF vaccine-associated viscerotropic disease (YEL-AVD), with the latter being less frequent but potentially fatal [5][6][7].
Safe vaccination and knowledge of how to respond to public concerns have a major impact on the success or failure of immunization programs. To this end, the analysis of cases and the dissemination of quality information for the recognition of post-vaccination adverse events, especially the serious and rare ones, strengthen the pharmacovigilance process in vaccines and provides subsidies for health professionals to deal with these events. Herein, we describe three cases of YEL-AND observed in 2021 following 17-DD vaccine (Instituto de Tecnologia de Imunobiológico, Bio-Manguinhos/FioCruz, RJ, Brazil) immunization in São José do Rio Preto, São Paulo State, Brazil.

Materials and Methods
Medical History, Sample Collection, and Diagnostic Analyses Herein, we describe three cases through retrospective analysis of clinical data (symptoms, laboratory, and radiologic observations) obtained from the medical records of meningoencephalitis-suspected pediatric patients admitted between July  According to the Brazilian Ministry of Health, the definition of YEL-AND-suspected cases, requires the presence of fever, seizures, neurological impairment associated with CSF alterations compatible to viral meningoencephalitis without other detectable causes, and within the first 30 days following yellow fever vaccination [7].

Case 1
A four-year-old female patient was admitted to the hospital on 25 August 2021 due to a month-long history of progressive symptoms: weight loss, hyporexia, emesis, and evolving with bradycardia and bradypnea. Brain computed tomography (CT) revealed signs of hypertensive hydrocephaly, and an external ventricular drainage was performed immediately thereafter. Brain nuclear magnetic resonance (NMR) imaging performed on the next day suggested an inflammatory-infectious process and a cerebral biopsy showed an area of granulomatous chronic inflammatory process with focal necrosis and histiocyte infiltration ( Figure 1a). Empirical treatment for bacterial, fungal, and viral meningitis were initiated. Due to the chronicity of the symptoms, the NMR, and biopsy findings with granulomatous characteristics, empirical treatment for neurological tuberculosis (TB) was also initiated. For that hypothesis, ceftriaxone 50 mg/kg/dose twice daily, fluconazole 12 mg/kg/day, and rifampicin 225 mg + isoniazid 150 mg + pyrazinamide 450 mg once daily were administered. A blood examination showed normal liver, hematological, and kidney functions.
The patient had received the second dose of the 17DD vaccine 36 days prior to the onset of symptoms and was hospitalized for 58 days, of which 24 days were spent in the intensive care unit (ICU). The external ventricular drainage was removed on day 21 post hospital admission. A secondary brain NMR showed significant improvement, and the patient was discharged on 22 October 2021. On follow-up 21 days after discharge, the patient showed no signs of symptom recurrence or any evidence of sequelae. Blood and CSF samples were submitted for screening against a comprehensive panel of infectious agents (Table 1). No agents were identified in serum, and CSF screening showed leukocytes 17 cells/mm 3 , lymphocyte predominance (86%), protein 153 mg/dL, glucose 36 mg/dL, and bacteria cultures and RT PCR for TB were negative. Enzyme The patient had received the second dose of the 17DD vaccine 36 days prior to the onset of symptoms and was hospitalized for 58 days, of which 24 days were spent in the intensive care unit (ICU). The external ventricular drainage was removed on day 21 post hospital admission. A secondary brain NMR showed significant improvement, and the patient was discharged on 22 October 2021. On follow-up 21 days after discharge, the patient showed no signs of symptom recurrence or any evidence of sequelae.

Case 2
A 10-month-old female was admitted on 7 July 2021 with a history of fever, vomiting for 6 days, progressive somnolence, lack of responsiveness, tonic-clonic seizures, and left hemiparesis. She had received the first 17DD dose 20 days prior to the onset of symptoms at 9 months and 7 days of age. The CSF analysis showed leukocytes 150 cells/mm 3 with a predominance of lymphocytes (72%), protein 110 mg/dl, and glucose 45 mg/dl. Culture for bacteria was negative. The patient was admitted to ICU, and a 50 mg/kg/dose of ceftriaxone twice daily and a 20 mg/kg/dose of acyclovir treatment thrice daily was initiated for probable bacterial and viral meningoencephalitis. Brain NMR imaging identified focal pachymeningeal enhancement in the frontal brain area. The electroencephalogram (EEG) showed bilateral degree I slowing, without epileptiform disorder ( Figure 1b). Symptoms were controlled and cleared following treatment, and the patient was discharged seven days following hospital admission. Subsequent to her discharge, CSF screening with a comprehensive ELISA panel for arboviruses, showed positive for anti-YFV IgM (Table 1). On clinical follow up 29 days after discharge from the hospital, the patient did not exhibit any recurring symptoms or sequelae.

Case 3
A 9-month-old female was admitted to the hospital on 6 August 2021 because of a week-long history of fever, hyporexia, and adynamia; a history of diarrhea, emesis, lethargy, and irritability whose onset presented fourteen days following the first dose of the 17DD vaccine. On admission, the patient received intravenous antibiotics (a ceftriaxone 50 mg/kg/dose bid and oxacillin 50 mg/kg/dose bid), and blood and CSF samples were collected. Analysis of CSF showed an increased leukocyte count (70 cells/mm 3 with lymphocytes 73% and monocytes 25%), protein 35 mg/dL, and glucose 51 mg/dL. Culture for bacteria was negative, anti-dengue IgM was negative, and anti-YFV IgM was positive using ELISA. In addition, brain NMR imaging indicated diffuse leptomeningeal enhancement ( Figure 1c). The detailed clinical data are shown in Table 1. The patient remained hospitalized for 6 days without the need for admission to the ICU and was discharged. On clinical follow-up 53 days after discharge, the patient did not exhibit any recurring symptoms or sequelae.

Discussion
The three reported cases of yellow fever vaccine-associated neurotropic disease (YEL-AND) following 17DD vaccination were detected by our health surveillance system. Although, the three events occurred in a short period, any relationship between the vaccine batch or vaccine administration establishment was not identified. Yet, none of the patients had a medical condition prior to the event or diagnosed during the follow-up. These case reports represent a rare phenomenon, albeit quite significant for public health safety.
Historically, the two live attenuated virus vaccines were developed in the 1930s to control YF disease. The 17D vaccine was developed in 1937 following 176 passages of the wild-type strain Asibi in mouse and chicken derived cell lines, and shortly thereafter the French neurotropic vaccine (FNV) was developed from a YFV strain isolated at the Institute Pasteur in Dakar (reviewed in [8]), and although effective, it was discontinued in the early 1980 s due to the high rate of post-vaccination complications [9]. Currently, only 17D-204 and 17DD are used around the world [3,8]. The administered 17DD vaccine is manufactured in Brazil by Bio-Manguinhos/Fiocruz, and it has been routinely used in several South American countries, while the 17D-204 vaccine is used outside Brazil and manufactured by Sanofi-Pasteur (France), Pasteur Institute Dakar (Senegal) and Federal State Budgetary Scientific Institution (Russia) [6,10]. Both are considered safe and effective, despite very rare cases of serious adverse events (SAEs) [8] commonly manifested as disorders affecting the immune response and the nervous system [11][12][13][14][15]. The risk of SAEs following immunization is higher in pregnant woman, thymus disorders, infants (6-8 months of age), or persons over 60 years of age [11][12][13][14][15]. Due to these risks, the Brazilian National Immunization Program applies the first dose of 17DD in infants at nine months of age, with a booster at four years of age. All three children described in this study received their first dose of the vaccine after reaching 9 months of age.
The development of YEL-AND may involve different mechanisms. For example, direct virus invasion may be responsible in meningoencephalitis syndromes, whereas in other neurological syndromes, such as Guillain-Barré syndrome and acute disseminated encephalomyelitis (ADEM), antibody and T-cell triggered autoimmunity may be involved [6]. The Brazilian Ministry of Health defines as a confirmed case of YEL-AND the presence of the following criteria: (a) symptom onset between one and 30 days after YF vaccination; (b) clinical presentation compatible with neurotropic disease; (c) CSF indicative of viral meningoencephalitis; (d) the exclusion of other causes; and (e) laboratory detection in CSF of the YFV vaccine by viral isolation, amplification, or by the presence of the anti-YFV IgM antibody [7]. Considering that the YFV IgM antibody does not cross the blood-brain barrier due to its high molecular weight, its presence in CSF may be considered as locally produced and indicative of CNS infection [14,16]. Beyond these criteria, the diagnosis of YF vaccine-related meningoencephalitis may be corroborated by complementary exams, such as neuroimaging.
In two cases presented here, the onset of symptoms occurred within the 30-day period following YF vaccination and were in line with the Brazilian Ministry of Health criteria for the definition of YEL-AND. In case 1, the onset of symptoms was mild and progressive around 30 days after vaccination, making it difficult to characterize the exact onset of symptoms. Additionally, all cases showed clinical symptoms of neurological syndromes compatible with meningoencephalitis (pleocytosis and increased protein level in CSF). YFV was the only possible cause confirmed by the presence of anti-YFV IgM antibodies.
We also carried out a short review using the main databases (PubMed, Lilacs, and Medline) using the keywords "YELLOW FEVER" AND "NEUROTROPIC" or "YELLOW FEVER VACCINE" AND "NEUROLOGICAL" AND " ADVERSE EVENT", resulting in 24 articles reporting several cases of YEL-AND summarized in Table A1 [11,. The data showed that adults and the elderly had a higher risk for development of SAEs, and the highest occurrence of SAEs were associated with the 17D-204 vaccine and presented after administration of the first dose [11,15,16]. In contrast, our cases were associated with administration of the 17DD vaccine, and in one of the cases SAEs developed following administration of the second dose of the vaccine.
Overall, YEL-related SAE reported rates are variable. Lindsey et al., 2016 [15], using historical data from the U.S. Vaccine Adverse Event Reporting System (VAERS) database from 2007 to 2013, reported a YEL-AND rate of 0.8/100,000 YF vaccinations (17D-204 (YF-VAX ® ), Sanofi Pasteur-Swiftwater, Pennsylvania). In parts of Africa, from 2007 to 2010, the reported incidence rate of YEL-AND was 0.016/100,000 doses of 17D-204 administered [18]. Lastly, in Brazil, between 2000 and 2015, the overall reported incidence rate of YEL-AND was 0.3/100,000 doses (17DD) administered, whereas in the period between 2007-2012, the rate was 0.17/100,000 doses administered [7]. It is so important to recognize its occurrence and understand its scale, especially for building a healthcare system able to identify and treat such cases. Moreover, a close examination of such cases reported in the literature and official bulletins might mitigate inappropriate notions about vaccine safety.

Conclusions
It is very important to emphasize that despite SAEs, the YF vaccine is safe, effective, and strongly recommended in risk areas with its benefits outweighing its risks. Moreover, the lethality from YFV infection is higher than the rates of SAEs triggered by vaccination. However, serious, and potentially fatal adverse events have been reported over the years, even though they are very rare. Thereby, it is imperative that healthcare personnel be prepared to identify such events, manage them early, and be able to critically and effectively communicate and educate the population of the benefits and the potential risks of vaccination in order to prevent misinformation and mitigate fears that may lead to reduced vaccination rates which could fuel future YFV outbreaks.